Electric pulse modulator



Jan. 3?, 1950 A. H. REEVES ELECTRIC PULSE MODULATOR 2 Sheets-Sheet 1 Filed April 5, 1948 F/GZ.

A tlorney Jan. 31, 11950 REEVES 2,495,768

ELECTRIC PULSE MODULATOR Filed April 5, 1948 2 Sheets-Sheet 2 FIG/Z f (a) b) Inventor Allorney Patented Jan. 31, 1950 2.49am usc'rmc PULSE uonum'ron Alec Harley Reeves, London, England, assignor to International Standard Electric Corporation,

New York. N. Y.,

a corporation of Delaware Application April 5. 1948, Serial No. 19,088 In Great Britain May 12, 1947 7 Claims.

The present invention relates to electric pulse modulating circuits employing cold cathode gasfilled discharge tubes.

The principal object of the invention is to reduce the operating cost and increase the life of pulse modulating circuits, by employing cold cathode gas-filled tubes as modulators. Such tubes require no heater supply, thereby avoiding a source of energy consumption, and n activated cathode surfaces which tend to lose their properties in time. The absence of any heater supply also facilitates the automatic switching on of the circuit by a control signal, since the only source which has to be switched is the high tension source.

The elimination of the heater supply is of particular importance in the case of portable equipment.

The invention provides an electric pulse time modulator comprising a cold cathode gas filled electric discharge tube having a principal discharge gap, means for applying to the gap a maintaining potential insumcient to initiate a discharge across the gap, means controlled by a modulating signal wave for periodically firing the gap at times depending on the instantaneous signal voltage, means for periodically extinguishing the discharge, and means for deriving time modulated pulses from the gap.

The invention will be described with reference to the accompanying drawings in which:

Figs. 1 and 3 show schematic circuit diagrams of two embodiments of the invention; and

Figs. 2 and 4 show wave form diagrams emplayed in explaining the operation of Figs. 1 and 3, respectively.

Fig. 1 shows a cold cathode tube l, filled with a suitable gas, or mixture of gases, at a suitable pressure, having an anode 2 and a cathode 3, both of which electrodes may be of any convenient and electrically suitable shapes. 4 is a second trigger anode preferably spaced at a smaller distance from cathode 3 than anode 2.

The anode 2 is connected through resistor 5 and one winding of signal-frequency transformer 5 to the positive terminal of battery 1, having its negative end grounded. Cathode 3 is connected to ground through variable resistor 9, shunted by condenser it.

The trigger anode 4 is connected through resistor i i and one Winding of signal frequency transformer i2 to a tapping on battery 1. Condenser l3 shunts the pulse frequency components but not the signal frequencies.

From a local source (not shown) negative rectangular pulses as shown at (a) Fig. 2, are applied at terminal I 4, and thence to the anode 2 through condenser 15, which is too small to shunt appreciably the signal components, but large enough'to pass the pulses. Battery 1 has a potential suflicient to maintain a discharge between electrodes 2 and 3 but not to start it, in the absence of a discharge from the trigger anode 4. Further the potential is high enough, under the given circuit and tube conditions, to prevent selfquenching of the discharge from 2 due to a squeg action when this anode is fired. As shown in Fig. 2 (a) the pulse voltage applied to the anode 2 is zero except during the time intervals A-B, C-D, E-F, etc., during which times it is sufliciently negative to quench rapidly any discharge between electrodes 2 and 3. The recurrence frequency of the pulses of Fig. 2 (a) should be at least twice that of the highest modulating signal frequency component; for speech of commercial quality it should be at least 8 k. c., and preferably at least 10 k. c.

From another local source (not shown) a repeated wave having the form shown in Fig. 2 (b) and the same recurrence frequency as the pulses, Fig. 2 (a) is applied to the anode 4 through resistor l6 and blocking condenser II. Between times AB, C-D, E-F, etc., it should be sufficiently negative to quench any discharge from the anode 4 to cathode 3; and from B to C, D to E, etc., it should rise linearly from a value V2 to a value Vi, which values are chosen in relation to the voltage of the tapping on battery I so that in the absence of voltage from transformer l2, trigger anode 4 will strike at a voltage V3, and at a time p approximately midway between times B and C (in the presence of a given voltage across condenser iii). The modulating signal voltage is applied at terminals 18 and to the primary winding of transformer I2, which should have a ratio suitable to match the impedance of the signal line to the impedance of the secondary winding circuit when anode 4' is not discharging. The circuit of anode 4 may be adjusted either (a) so that once anode 4 is fired it continues discharging until the local wave of Fig. 2 (b) quenches it at times A, C, E etc., or alternatively (b) so that once fired, anode 4 immediately starts a squeg and quenches itself in the mannner explained in the specification of co-pending U. S. application Serial No. 19083/48, filed April 5, 1948, the squeg time-constant being arranged so that the charge accumulated in condenser 17 during the squeg has died away substantially to zero by times B, D, F, etc. The gas or gas mixture, gas pressure, and

other tube and circuit constants are designed so that-at times B. D,'F, etc., the tube hasbecome restored sufficiently to enableit to retire, if necessary, from both anodes'land 4. Q

. The slowly rising curve of, Fig. 2 shows a typical portion of the modulating signal voltage after arrival at anode 4. The sum of 2 (b) and 2 (c), as shown by curve 2 (d), will represent the total voltage applied to the anode 4 due to the signal and the local wave 2 (b). As shown, it will cross the line of voltage V3, at which the trigger anode 4 will fire, at times q, r, s, etc., On account of the rising wave of the signal it is clear that the time intervals Bq, D-r, F-s, will progressively decrease; and therefore that the time durations of the current pulses from anode 2, fired simultaneously by trigger anode 4 at these times q, r, s, and quenched at the fixed times, A, C, E, will progressively increase. Anode 2 will thus produce current pulses shown by curve 2 (e) which are duration modulated by the signal. These signal modulated pulses may therefore be taken from transformer 6 at terminals l9.

According to well-known principles, the linearity of the modulation may be improved if necessary, at the expense of gain, by adding a given amount of negative feed-back. This is achieved by resistor 9, shunted by condenser Ill. The time constant of elements 9, I0 should be adjusted so that the pulse-frequency components are substantially shunted by [0, but not the signal frequencies. In this case, as the output rises, the signal-frequency voltage across ID will rise, which will tend to delay the firing of anode 4, and thus to reduce the output. The action in reducing distortion components is exactly similar to that of the usual negative feed-back amplifier.

The negative feed-back obtained is proportional to the value of resistor 9. If full gain and no feed-back is required, resistor 9 is reduced to zero.

A modification of the circuit of Fig. 1 which does not employ any locally generated waves is shown in Fig. 3. Many of the elements are the same as in Fig. l, and have been given the same designation numbers, and will not be again described. Elements M to IT have been omitted. A resistor 20, shunted by a condenser 2| has been added in series between the resistor 9 and the cathode 3.

The constants, including the values of elements 20 and 2| are arranged so that as soon as the anode 2 fires after the trigger anode 4 has been fired by the signal voltage applied to terminals l8, the current flowing to cathode 3 starts a squeg in the manner explained in the specification of co-pending U. S. application Serial No. 19083/48, filed April 5, 1948, which extinguishes the discharge to both anodes 2 and 4.

Condenser 2| should be very small (only a few micromicrofarads.) and the time constant of elements 20, 2| should be adjusted so that the condenser always discharges sufiiciently for anode 4 to be refired after a time small compared with one period of the highest applied signal component. The actual moment at which anode 4 will be refired depends on the momentary value of the signal voltage. Thus if the signal wave is that shown in Fig. 4 (a) anodes 4 and 2 will be refired progressively earlier after each squeg. The resulting voltage across the condenser 2! is shown in Fig. 4 (b). The result is a frequency modulation of the squeg by the signal, as shown. It the tube did not retire after the first time, the voltage across the condenser would follow the curve ABC-of-Fig. 4 '(b), falling off to zero exponentially. As infact it refires, it follows the curve AB and then the curve BD, and similarly for succeeding cycles. Typical wave forms of the resulting current pulses through anode 2 are shown in Fig. 4 (c).

The duration and shape of each pulse will be substantially constant, but the time interval between successive pulses will be modulated in accordance with the applied signal voltage. The frequency modulated pulses may be obtained as before from terminals [9.

The tube l of Figs. 1 and 3. preferably contains 92% neon, 1% argon and 7 hydrogen at a total pressure of mm./Hg. The three electrodes are preferably pure nickel rods 5 mm. long and 1 mm. in diameter, rods 2 and 3 being arranged end to end in line with a 5 mm. gap between them, anode 4 also being parallel to 3, and spaced 5 mm. from 2. In this case, the time interval A-B of Fig. 2 (a) may be reduced to about 5 microsecs., allowing for effective handling of signal frequencies up to about 50 k. c. The circuit of Fig. 3 with the same tube gives a minimum squeg period of about 6 microsecs. (before refiring) and if 2:1 be taken as the minimum useful ratio between maximum and minimum squeg periods, then signal frequencies of up to about 40 k. c. could be usefully handled. In both cases, such a limiting frequency would be amply sufficient for both speech and music.

It should be noted that both circuits, Figs. 1 and 3, produce pulses which are'modulated in respect of a time characteristic. In Fig. l the characteristic is the pulse duration, and in Fig. 3 it is the period between pulses.

What is claimed is:

1. An electric pulse time modulator comprising a cold cathode gas-filled electric discharge tube having a principal discharge gap, means for applying to the gap a maintaining potential insufiicient to initiate a discharge across the gap, means controlled by a modulating signal wave for periodically firing the gap at times depending on the instantaneous signal voltage, means for periodically extinguishing the discharge, and means for deriving time modulated pulses from the gap.

2. An electric pulse duration modulator comprising a cold cathode gas-filled electric discharge tube having a cathode; a principal anode, and a trigger anode forming a principal gap and a trigger gap, means for applying across the principal gap a maintaining potential insufficient to initiate a discharge thereacross, means for applying between the trigger anode and the cathode a train of regularly repeated voltage waves of saw-tooth form together with a modulating signal voltage wave in such manner as to fire the trigger gap at a.time during each period of the saw tooth waves depending on the signal voltage, means for periodically extinguishing the discharges across both gaps, and means for deriving duration modulated pulses from the principal anode.

-3. An electric pulse frequency modulator comprising a cold cathode gas-filled electric discharge tube having a cathode, a principal anode, and a trigger anode forming a principal discharge gap and a trigger discharge gap, means for applying across the principal gap a maintaining potential insufficient to initiate a discharge thereacross, a

with a polarising voltage across the trigger gap, the arrangement being such that the combined voltage is sumcient to fire the trigger gap thereby causing the principal gap to fire and extinguish both discharges by squegging, in such manner that the charge acquired by the condenser prevents reflring of the trigger gap for a period determined by the time constant of the said resistance and condenser, and means for deriving frequency modulated pulses from the principal anode.

4. A modulator according to claim 2 comprising means for applying to the principal anode a train of negative pulses regularly repeated at the same frequency as the saw tooth waves for periodically extinguishing the discharge cross the principal gap.

5. A modulator according to claim 2 in which each saw tooth wave is followed by a short nega- 20 Number amplitude to xtinguish the discharge across the trigger gap.

tive pulse oi suflicient 6. A modulator according to claim 2 in which" ing potential.

ALEC HARLEY REEVES.

REFERENCES CITED The following references are: of record in the file of this patent:

UNITED STATES PATENTS Name Date 1,875,151 Rentschlen Aug. 30. 1932 1,898,486 Hund Feb. 21, 1933 2,441,958 De Rosa.-

May 25, 1948 Q 

